One of the major objectives of coal combustion research is the development of comprehensive computer models to help design combustors and gasifiers for the clean utilization of coal usually in complex burners and combustion chambers. Coal combustion is the process of combination of different processes which proceed at different rates and mutually interdependent of one-another (Fig. 5.1).
Figure 5.1: Schematic presentation of a typical combustion process of a single coal particle.
A few qualitative remarks about the combustion of coal particles are made in the previous chapters. This aspect will now be dealt with in a more general fashion following the mathematical analysis and several other significant works. In order to adequately describe the combustion of coal in a fixed or fluidized-bed or in other industrial applications, it is essential that the mechanism which governs the combustion of a single coal particle is well understood. Only limited effort has been made to resolve this problem and such mechanistic models will be detailed in this section.
First, a brief discussion will be presented of the general features and main issues which have made it difficult or even impossible to unambiguously describe the coal particle combustion phenomenon. Bywater has presented an order of magnitude time scale for the different physical and chemical operations that occur in a fluidized-bed combustor.
On the other hand gas may take a relatively much shorter time around 1 sec to pass through the bed. As explained in the earlier sections, the volatiles may take typically 1-10sec to be released from the coal but may burn much faster in less than 1 sec. Borghi et al. found that the burn out time of char is two orders of magnitude longer than the time for the combustion of volatiles. Consequently the former controls the burning of coal in a fluidized-bed combustor. The burning of the devolatilized coal particles involves the conversion of the fixed carbon of coal into CO and CO2. A quantitative description of this process involves complete resolution of many involved issues and these are elaborated elsewhere [Basu].
The burning rate of a char particle is controlled by the diffusion of gas from the surroundings to the char surface and is referred to as external transport. The process is characterized by the value of an effective diffusion coefficient. The actual value of effective diffusion coefficient amongst other factors will depend upon porosity and tortuosity of the voids inside the particle. The diffusion of gas through the pores inside the char and the chemical reaction on the pore walls is referred to as intrinsic reactivity.
For low intrinsic reactivity the oxygen is able to travel into the interior of the char particle. For such a case, the particle size stays constant during combustion but its density decreases. On the other hand if the reaction rate is very fast, all the oxygen is consumed as it reaches the particle surface. For such a case, the density of the particle stays constant while the particle size changes.
Several workers have attempted to identify the nature of products formed at the particle surface on the basis of what is now known as the two-film model [Hougen et al.] or continuous film model a part of the present work. It is assumed that the oxygen
from air diffuses to the surface of the particle and reacts with the carbon of the coal to form CO and CO2. The chemical reactions are represented in the Figure 5.2.
CO O
C+12 2 → , (1)
. 1
396 −
−
=
ΔH kJmole
2
2 CO
O
C+ → , (2)
. 1
113 −
=
ΔH kJ mole .
One can conclude that the extent to which the second reaction occurs at the particle surface is negligible and predominantly the first reaction takes place.
Two Film Model:
CO CO
C+ 2 →2
2 2 2
1O CO
CO+ →
Continuous Film Model:
CO CO
C+ 2 →2
2 2
2
1O CO
CO+ →
2
2 CO
O C+ →
CO O
C+12 2 →
Figure 5.2: Combustion model for the burning of a coal particle: concentration profiles of different species.
CO thus formed at the interface diffuses outwards into the air stream where it reacts with the incoming oxygen to form carbon dioxide in the homogeneous gas-phase reactions,
2 2
2
1O CO
CO+ → , (3)
. 1
281 −
−
=
ΔH kJ mole .
The burning of CO is catalyzed by the presence of traces of water vapor and, thus, its chances to escape in the gas stream are rather scarce. The burning of CO in a diffusion flame and the existence of the reaction zone has been experimentally demonstrated.
Carbon di-oxide thus formed diffuses back to the particle surface to undergo the heterogeneous reduction reaction at the particle surface,
CO CO
C+ 2 →2 , (4)
. 1
5 .
172 −
=
ΔH KJmole .
CO thus generated diffuses outwards and combines with the incoming oxygen to form CO2. The rest of the cycle repeats again and goes on and on to sustain continuous combustion. The above mentioned qualitative mechanisms of the burning of a coal particle are shown in Figure 5.2.
According to the two-film model, it is claimed that the controlling mechanism for carbon combustion will have the reactions of carbon with carbon dioxide and of carbon monoxide with oxygen which is fast enough. Under this condition, hardly any oxygen will reach to the surface of the coal particle so that the mechanism for the carbon combustion based on the stipulation of the direct oxidation of carbon will not be possible. Avedesian and Davidson developed a quantitative mathematical model for char combustion in a fluidized bed based on the above qualitative description of char particle combustion. Many researchers have argued that when the oxygen of air comes in contact with the carbon of the particle, both CO and CO2 are produced. However, if the temperature is greater than about 923 K (the ignition temperature of CO), the CO burns in a reaction zone surrounding the particle (direct oxidation model [Basu et al.]. If the particle temperature is above 1373 K, CO2 is reduced to CO on the particle's surface.
Basu et al. mention that the direct oxidation model is relatively more valid for char combustion than the two-film model under conditions which are characterized by low particle Reynolds number and high temperatures in the range 1173-1573 K.
In order to establish the combustion mechanism for a given combustion system, it is necessary to ascertain two important facts. First, what are the combustion products on the char particle's surface? Secondly, which of the two gases, oxygen or carbon dioxide,
can preferentially diffuse to the surface? Many workers such as Meyer, Strickland- Constable, Shah, Sihvonen and Arthur have attempted to answer the first question.
Arthur on the basis of his experiments on carbon burning from artificial graphite and a coal char concluded that depending upon the states of carbon and oxygen molecules both CO and CO2 are produced. The effort of Basu et al. was motivated by the aim to investigate whether the conclusions drawn by Arthur and others, in connection with relative proportions of CO and CO2 generated on the carbon particle surface under the gas flowing condition, are also valid when such particles are burned in a fluidized bed or not. Basu et al. examined the combustion of 1-3 mm diameter anthracite coal particles in a fluidized bed (129mm diameter) at 1123 K and for air fluidizing velocity in the range 0.2-0.3 m/sec. The experiments indicated that the primary combustion products were both CO and CO2, and oxygen did diffuse to the carbon surface.
At about 1100 K, the diffusion coefficient of oxygen is slightly larger than that of carbon dioxide. Basu et al. comment: "If the oxidation rate of CO is much higher than the diffusion rate of O2, all CO will be quickly consumed leaving the remaining oxygen to diffuse to the surface." They concluded that further experimentation is required to establish the conditions in terms of the size and temperature for transition from the two film mechanism to continuous film mechanism. Avedesian and Davidson and Campbell and Davidson computed the concentration profiles for the two-film model as shown in Fig. 5.2. They assumed that the oxidation reaction of CO (Eq. (3)) consumed all the oxygen which diffused to the carbon surface. There is contradiction to this hypothesis on two grounds. First, the reaction of Eq. (3) is not infinitely fast; and second, the endothermic reaction of Eq. (4) cannot receive sufficient heat from the reaction zone. It is, therefore, concluded that oxygen diffuses to the particle surface and the entire CO produced on it burns in a reaction zone away from the surface.
In this chapter, a mathematical model to describe the combustion of a single coal particle is presented. Combustion modeling is divided into two section i.e. gas phase modeling and solid phase modeling. An assumption of continuous film oxidation for CO is taken into consideration. In order to model solid phase which consists of a number of pores, the effective values of many physical parameter are taken into consideration.
Furthermore, a mathematical model using the new values of the reaction coefficient (chapter 4) to model a special case of combustion where oxygen is not available in excess is illustrated.